Spatial light modulator having an analog beam for steering light

ABSTRACT

A spatial light modulator (40, 70, 80) operable in the analog mode for light beam steering or scanning applications. A deflectable mirror (42, 72) is supported by a torsion hinge (44) along a torsion axis. A plurality of flexure hinges (48) are provided to support the corners of the mirror (42, 72) and provide a restoration force. The combination of the torsion hinges and the flexure hinges realizes a deflectable pixel that is operable in the linear range for a large range of address voltages. The flexure hinges also maintain a flat undeflected state when no address voltage is applied, and prevent the pixel from collapsing. The pixel may be reinforced, such as about its perimeter (74) to ensure mirror flatness and prevent warping, even during extreme deflections of the mirror. The pixel is electrostatically deflected by applying an address voltage to an underlying address electrode (60).

    ______________________________________                                                                      FILING                                          SER. NO.  TITLE               DATE                                            ______________________________________                                        08/414,831                                                                              Spatial Light Modulator with                                                                      03-31-95                                                  Superstructure Light Shield                                         08/097,824                                                                              Microminiature, Monolithic,                                                                       07-27-93                                                  Variable Electrical Device and                                                Apparatus Including Same                                            08/424,021                                                                              Active Yoke Hidden Hinge                                                                          04-18-95                                                  Digital Micromirror Device                                          08/171,303                                                                              Multi-Level Digital Micromirror                                                                   12-21-93                                                  Device                                                              ______________________________________                                    

FIELD OF THE INVENTION

The present invention is generally related to spatial light modulatorsfor modulating incident light, and more particularly, to a device havinga selectively deflectable mirror being supported by at least one hingefor steering incident light in a direction being a function of thedegree to which the mirror is deflected.

BACKGROUND THE INVENTION

Spatial light modulators (SLM's) have found numerous applications in theareas of optical information processing, projection displays, video andgraphics monitors, televisions, and electrostatic printing. SLM's havealso found uses as optical switches, optical shutters, light valves,pixel steerers and so forth. SLM's are devices that modulate incidentlight in a spatial pattern to form a light image corresponding to anelectrical or optical input. The incident light may be modulated in itsphase, intensity, polarization, and/or direction. The light modulationmay be achieved by a variety of materials exhibiting variouselectro-optic or magneto-optic effects, and by materials that modulatelight by surface deformation.

The present invention relates to SLM's of the foregoing type which maybe used in a variety of devices, including light switches, light valves,pixel steerers and optical shutters.

A recent innovation of Texas Instrument Incorporated of Dallas, Tex. isthe digital micromirror device or deformable mirror device, collectivelyknown as the DMD. The DMD is a spatial light modulator comprising amonolithic single-chip integrated circuit, typically having a highdensity array of 17 micron square deflectable micromirrors but may haveother dimensions. These mirrors are fabricated over address circuitryincluding an array of memory cells and address electrodes. The mirrorsmay be bistable and be operated in the digital mode, the mirror beingstable in one of two deflected positions. A source of light directedupon the mirror is reflected in one of two directions by the mirror.When used in the digital mode, incident light from the array of mirrorscan be modulated and directed to a projector lens and then focused on adisplay screen or a photoreceptor drum to form a light image. When themirror is in one position, know as the "on" position, light is directedinto the projector lens. In the other position, know as the "off" mirrorposition, light is directed to a light absorber. The DMD may also bemonostable and operated in the analog mode, and finds use as a lightswitch, pixel steerer, optical shutter, scanner, and the like.

For a more detailed discussion of the DMD device and systemsincorporating the device, cross reference is made to U.S. Pat. No.5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method",U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard IndependentDigitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled"Printing System Exposure Module Alignment Method and Apparatus ofManufacture", each patent being assigned to the same assignee of thepresent invention, and the teachings of each are incorporated herein byreference. Gray scale of the pixels forming the image may be achieved bypulse width modulation techniques of the mirrors, such as that describedin U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing forUse in a Pulse-Width Modulated Display System", assigned to the sameassignee of the present invention, and the teachings of each areincorporated herein by reference.

Commonly assigned U.S. Pat. No. 4,662,746 to Hornbeck entitled "SpatialLight Modulator and Method", 4,710,732 to Hornbeck entitled "SpatialLight Modulator and Method", U.S. Pat. No. 4,956,619 to Hornbeckentitled "Spatial Light Modulator", and U.S. Pat. No. 5,172,262 toHornbeck entitled "Spatial Light Modulator and Method" disclose variousstructures and methods of producing micro mechanical devices,specifically, monostable DMD SLM's suited for use in the analog mode,the teachings of each incorporated herein by reference.

Commonly assigned U.S. Pat. Nos. 5,096,279 to Hornbeck et al. entitled"Spatial Light Modulator and Method", U.S. Pat. No. 5,142,405 toHornbeck entitled "Bistable DMD Addressing Circuit and Method", and U.S.Pat No. 5,212,582 to Nelson entitled "Electrostatically Controlled PixelSteering Device and Method" disclose various structures and methods forproducing the same that are bistable and suited for use in the digitalmode, the teachings of each incorporated herein by reference.

Referring to FIGS. 1A-1H, these embodiments being disclosed in acommonly assigned U.S. Pat. No. 5,172,262, there is shown a monostableDMD spatial light modulator that can be operated in the analog mode. Onepixel, generally denoted at 20, is basically a flap covering a shallowwell and includes a silicon substrate 22, a spacer 24, a hinge layer 26,a pixel layer 28, a flap 30 formed in layers 26-28, and plasma etchaccess holds 32 in flap 30. The portion 34 of hinge layer 26 that in notcovered by pixel layer 28 forms a hinge attaching flap 30 to the portionof layers 26-28 supported by spacer 24. Pixel 20 is fabricated using arobust semiconductor process upon silicon substrate 22. Spacer 24 may bean insulating positive photoresist or other polymer, hinge layer 26 andpixel layer 28 are both an aluminum, titanium and silicon alloy (Ti: Si:AL), although these layers could also comprise of titanium tungsten, orother suitable materials. The hinge layer 34 may be about 800 Angstromsthick, wherein the pixel 30 is much thicker to avoid cupping andwarping, and may have a thickness of about 3,000 Angstroms.

Pixel 20 is operably deflected by applying a voltage between mirror 30and an underlying address electrode 36 defined on substrate 22. Flap 30and the exposed surface of electrode 36 form the two plates of an airgap capacitor, and the opposite charges induced on the two plates by theapplied voltage exert electrostatic force attracting flap 30 tosubstrate 22. This attractive force causes flap 30 to bend at hinge 34and be deflected toward substrate 22. Depending on the opposing surfacearea of the electrodes, the spacing therebetween, the differentialvoltage applied, and the compliance of hinge 34, the degree ofdeflection of mirror 30 will vary. The deflection of mirror 30 is afunction of the differential voltage, as graphically illustrated in FIG.1C. As shown, the greater the differential voltage, that is, the greaterthe voltage applied to mirror 30, the greater the degree of deflection.

As also illustrated in FIG. 1 C, this deflection is nonlinear, and isnot proportional to the voltage applied. A linear response, which may bethe ideal response, is shown by the dotted line generally depicted at38. The nonlinear relationship is due to many reasons. First, theelectrostatic force is a function of the inverse of the square of thedistance separating the mirror 30 and address electrode 36. Secondly,the geometry and composition of the hinge affects the compliance ofhinge 34. The thickness of mirror 30 prevents significant warping, butthe thinness of hinge 34 allows for large compliance. The deflection offlap 30 is a highly nonlinear function of the applied voltage becausethe restoring force generated by the bending of hinge 34 isapproximately a linear function of the deflection, but the electrostaticforce of attraction increases as the distance between the closest cornerof flap 30 and electrode 36 decreases. Recall that the capacitanceincreases as the distance decreases so the induced charges both increasein quantity and get closer together. As shown in FIG. 1C, the voltage atwhich mirror 30 becomes unstable and bends all the way to touch andshort with electrode 36 is called the collapse voltage. The analogoperating region is that region between zero deflection and thecollapsed situation.

FIG. 1D-1H illustrate equivalent alternative embodiments of thecantilever or leaf-type mirror 30 shown in FIG. 1.

When operating a spatial light modulator, such as of the type justdiscussed and referenced in FIG. 1A-1H, it may be desired to operatedthe deflectable member in the analog region, whereby the angle ofdeflection of mirror 30 is linearly proportional to the voltage applied.To operate the device as a light beam steerer, scanner, or light switch,it is desirable to precisely control the degree of deflection as so toprecisely steer incident light to a receiver, such as a sensor.Therefore, in prior art designs, such as that shown in FIGS. 1A-1H, itis imperative that a repeatable process be followed. In the practicalworld, however, process tolerances allow for some deviation form deviceto device. Thus, for a given voltage being applied to address electrode36, the deflection of mirror 30 from device to device will varyslightly. Consequently, characterization of the device prior toimplementation is necessary when the device is used in the analog mode.

It is desired to provide a spatial light modulator with a deflectablepixel well suited to be used in the analog mode. The deflection of thepixel as a function of the address voltage should be easilycharacterized, consistent from device to device even with processtolerances, and approximately linear throughout a large range ofdeflection.

SUMMARY OF THE INVENTION

The present invention achieves technical advantages as a spatial lightmodulator by providing a pixel supported by both a torsion hinge and atleast one flexure hinge. The torsion hinges define the tilt axis, andthe flexure hinges define the flat position and provide a restoringforce to achieve enlarged stable range of tilt angles. Through thecombination of these hinges, the pixel is nearly monostable and willresist collapse. The pixel structure may be reinforced to help maintainmirror flatness.

The present invention comprises a spatial light modulator of the typewhich includes a generally planar light-reflecting pixel, i.e. mirror,which is deflectable out of a first, normal position into a plurality ofsecond positions. Light incident on the pixel is selectively modulatedby being reflected from the pixel in selected directions, depending onthe position of the pixel. The position of the pixel is dependent on aselected characteristic of an electrical signal, such as a voltageapplied to an underlying address electrode. The deflection of the pixelstores potential mechanical energy in a pixel supporting facility, thisstored energy tending to return the pixel to the first, horizontalnormal position. Preferably, this pixel-supporting facility includes afirst torsion hinge connected between the pixel and the first stationarypost and defining a torsion axis. Deflection of the pixel effects itsrotation about the torsional axis of the first hinge. At least onesecond flexure hinge is connected between the pixel and a secondstationary post. Deflection of the pixel effects the flexure of thesecond hinge. By providing two types of hinges, deflection of the pixelis controllable for a large deflection range, and approximatelyproportional to the electrical signal applied to the underlining addresscircuit, preferably being an electrode. The pixel is monostable andcannot collapse on the address electrode, unless of course, largeaddress voltages are provided. Excellent accuracy of the pixel mirrortilt angle for light steering is achieved, with the second hingeproviding a well-defined undeflected (flat) position. Both the firsthinge and the second hinge provide a restoration force.

The points of connection of the first and second hinges to the pixel areseparated about the perimeter of the pixel. The pixel has a generallyorthogonal profile, with the first hinge being connected to the pixel ata mid-point of a first side thereof. The second hinge is connected tothe first side of the pixel as well in one embodiment. Preferably, thesecond hinge is connected to the pixel proximate the juncture of thefirst side of pixel and a second side of the pixel. The second hinge maybe connected to either the first or second side of the pixel. Thetorsional axis of the first hinge is generally co-planar with the pixel,and the second hinge in its unflexed state is oriented so as to begenerally co-planar with the pixel and perpendicular to the torsionalaxis of the first hinge. The second hinge in its flexed state defines acurved or slightly twisted surface at its corner, such that each endremains co-planar with the surface to which it is attached.

The characteristic of the first hinge is such that the deflection of thepixel out of the first position is predominantly rotational about thetorsion axis. The characteristic of the second hinge is such that thefirst position of the pixel is predominantly determined by the secondhinge. The characteristics of both the hinges are such that the pixel isselectively deflectable out of the first position into a plurality ofsecond positions determined by the selected characteristic of theelectrical signal. The first and second hinges are respectivelyconnected to the separated first and second post. In an alternativeembodiment, the first and second hinges may be connected to the samepost. The pixel may be reinforced to maintain flatness when undeflected,and when deflected out of the first position. Preferably, thereinforcement may comprise the perimeter of the pixel being corrugatedor ridged to provided rigidity and minimize interfering with incidentlight being modulated. Alternatively or additionally, cupping of thepixel may be minimized by providing radial corrugations or ridges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1H illustrate a prior art cantilever-type spatial lightmodulator, including a deflectable pixel deflectable as a function of anaddress voltage applied to an underlying address electrode;

FIG. 2 is a perspective view of a spatial light modulator according tothe present invention including a deflectable pixel supported both by atorsion hinge, and at least one flexure hinge connected proximate thecorners thereof;

FIG. 3A is a side view of the SLM of FIG. 2 with the torsion hinge postsremoved, illustrating the pixel being deflected about the torsion hingeto steer incident light in a selected direction, the deflection of thepixel being a function of the voltage applied to the underlying addresselectrode;

FIG. 3B is a graph illustrating the linear deflection of the mirror as afunction of the addressed voltage.

FIG. 4 is a top view of an alternative preferred embodiment of the pixelshown in FIG. 2, whereby the perimeter of the pixel is reinforced toinsure the pixel is flat, rigid, and remains unflexed, even when pivotedabout the torsional axis;

FIG. 5 is a cross section taken along line 5-5 in FIG. 4, illustratingthe reinforcing structure of the pixel comprising the pixel beingcorrugated about the perimeter; and

FIG. 6 is a top view of yet another alternative preferred embodimentwherein the torsion hinges and flexure hinges are supported by a commonpost.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, there is generally shown at 40 a monostable spatiallight modulator according to the preferred embodiment of the presentinvention. SLM 40 is well suited to operate in the analog mode toselectively steer incident light in a direction being a function of anelectrical input. The deflection of the SLM may be generally linear ornonlinear, as a function of the input electrical signal. The pixelresists collapse on its underlying address electrode due to the uniquecombination of hinges.

SLM 40 is a micromechanical structure formed using robust semiconductorprocessing. SLM 40 is seen to comprise a generally rectangularreflective aluminum pixel 42 being supported along its mid section by apair of torsional hinges 44. Hinges 44 essentially bisect the mirror 42and define a torsional axis therealong. Mirror 42 is also seen to besupported proximate each corner thereof by an L-shaped flexure-typehinge 48. Torsional hinges 44 extend from and are supported by arespective electrically conductive post 50, with the flexure hinges 48being supported by a respective electrically conductive post 54. Each ofposts 50 and 54 are supported upon a silicon substrate 56, thissubstrate 56 also supporting a pair of electrically conductive addresselectrodes 60. Each of address electrodes 60 are connected tounderlining address circuitry fabricated within substrate 56 (notshown). Hinges 44 connect to mirror 42 in a pair of opposed notches 58,notches 58 permitting long and compliant hinges 44. While the hinges,mirror and support posts are preferably comprised of electricallyconductive material, each or all could also be comprised of electricallynon-conductive material with the pixel having a light-reflective coatingif desired. Hinges 48 could also have a serpentine shape, or be linearif desired. Hinges 48 could connect to pixel mirror 42 or either sideproximate the corner of the adjoining sides, and limitation to theillustrated shapes of hinges 48 is not to be inferred.

Referring to FIG. 3A the angular deflection of mirror 42 about thetorsional axis defined by hinges 44 is seen to be a function of thevoltage potential applied to one of the address electrodes 60. With abias voltage being applied to mirror 42 via posts 54 and hinges 48, andan address potential being applied to one of the two address electrodes60, this voltage potential induces an electrostatic attraction betweenthe mirror 42 and the underlying address electrode 60, thus creating anangular deflection of mirror 42, as shown. Torsional hinges 44 rotate ortwist with mirror 42 and provide restoring force in the form of amechanical energy. Each of the four flexure hinges 48 also provide arestoring force, and deform or flex, as shown, when mirror 42 deflectsabout the torsional axis defined by hinges 44. Hinges 48 also providerestoring force in the form of mechanical energy, and define a normalflat or undeflected position when no voltage potential exists.

By way of illustration but with no limitation to the followingdimensions or shapes, torsional hinges 44 are preferably comprised of acompliant material such as aluminum, an aluminum alloy or titaniumtungsten, each having a thickness of about 500 Angstroms. Each offlexure hinges 48 are also comprised of a compliant metal, such asaluminum, an aluminum alloy, or titanium tungsten, each having athickness of about 500 Angstroms. The length of each hinge 48 isapproximately 10 microns. Each hinge 48 extends from the respective post54 a substantial length towards a respective torsional hinge 44 and isperpendicular therewith. Each flexure hinge 48 has a 90° bend proximatemirror 42, and is connected to the corner of pixel 42, at the junctureof two adjacent sides, as shown. The short segment of hinge 48 insuresthat a majority of the flexure of hinge 48 is along the major length,with any twisting taking place at the corner thereof. With the hinge 48being about 500 Angstroms in thickness and having a length of about 10microns, the flexure of these hinges permits mirror 42 to deflect as afunction of the voltage potential provided to one address electrode 60,as shown in FIG. 3A. The spacing of mirror 42 from electrodes 60 isabout 1-10 microns. This analog operating range is represented as anglee, as shown in FIG. 3A. This corresponds to an input voltage of between0 and 20 volts. As pixel 42 deflects angle θ, incident light is steeredthrough a range of 2θ, as shown in FIG. 3A. As expected due to opticallight properties, the angular range that incident light can be reflectedis double the angular deflection of the pixel 42. In the presentinvention, pixel 42 can be deflected with a linear or non-linearresponse, depending on the design, up to angle being about 10°. Thus,the range of steering light is 20°. The response curve of mirror 42 as afunction of address voltage is shown in FIG. 3B.

With an address voltage being applied to one address electrode 60 beingfrom 0 to 20 volts, mirror 42 is deflected proportional to the addressvoltage. When SLM 40 is operated as an optical switch or light steerer,incident light can be precisely steered to a receiver such as an opticalsensor or scanner. The mirror tilt angle can be achieved with aexcellent accuracy for pixel steering. The torsion hinges 44 define thetilt axis, whereby the flexure hinges 48 help achieve a controlledresponse and maintain mirror levelness when not addressed (undeflected).Both the torsion hinges and flexure hinges provide a mechanicalrestoration force and achieve a stable tilt range. With two flexurehinges 48 being provided each side of the torsion axis, there is littlepossibility of a collapsed mirror, unless one of the hinges shouldbreak, which is not likely given the range of operable address voltagesprovided to address electrode 60. Compliance of each of flexure 48 andtorsion hinges 44 is excellent.

Referring now to FIG. 4, a top view of an alternative preferredembodiment of the present invention is shown as SLM 70, with a modifiedmirror 72 is shown, wherein like numerals refer to like elements of thefirst embodiment. To ensure that mirror 72 remains flat and does notwarp, even in an extreme deflected state, the perimeter of mirror 72 isreinforced. This reinforcement preferably is achieved by corrugating theperimeter of the mirror, shown as a trench shown at 74. Referring toFIG. 5, a cross section of mirror 42 taken along line 5--5 in FIG. 4illustrates how mirror 42 is corrugated about the perimeter thereof. Thetrench 74 in the metal is one reliable way to reinforce mirror 72 toprevent warping or cupping, even when deflected about the torsion axis.Other equivalent methods of reinforcing could include increasing thethickness of mirror 72 about the perimeter thereof, like a rib or ridge,and grid.

Referring now to FIG. 6, another alternative embodiment of the presentinvention is shown, as SLM 80, wherein like numerals refer to likeelements. Each of flexure hinges 82 are connected to one of posts 84from which torsion hinges 86 extend. This embodiment requires only twosupport posts as compared to the six support posts shown in FIG. 2.Again, each of the flexure hinges 82 extend from the respective posts 84and connect to one corner of mirror 72, proximate the juncture of twoadjacent sides. All the hinges provide a restoration force to return themirror to a flat, undeflected state when no address voltage is appliedto address electrode 60. This embodiment permits a higher fraction ofoptically active surfaces since SLM elements can be placed closertogether.

In summary, an improved monostable spatial light modulator is disclosedthat is well suited to operate in the analog mode. Deflection of thepixel is proportional to an electrical signal applied to an underlyingaddress electrode. The deflection and response of the pixel operates isselectable throughout a large range of address voltages. Thus, anincident light beam can be precisely steered to a receiver, such as asensor or a light scanner. Given the tight process parameter tolerancesof conventional semiconductor processing, spatial light modulators withrepeatable operating characteristics can be manufactured. The flexurehinges ensure the mirror will resist collapse, contribute to themonostable analog operating characteristics of the mirror, provide arestoring force, and establish mirror levelness in the unaddressedsituation. The torsional hinges define the tilt axis, and also provide arestoring force. The present invention can be manufactured using robustmanufacturing processes utilized in manufacturing monolithic integratedcircuits. The present invention is inexpensive, light weight, andrequires low drive power. The deflection speed of the device is veryhigh, with response time of the mirror deflection well suited for highspeed optical switching. The surface area of the mirror can be customdesigned to have a very small size, such as 17 microns square, but canbe relatively large if desired to provide a large area flat pixel forsteering large-area light beams.

Though the invention has been described with respect to a specificpreferred embodiment, many variations and modifications will becomeapparent to those skilled in the art upon reading the presentapplication. It is therefore the intention that the appended claims beinterpreted as broadly as possible in view of the prior art to includeall such variations and modifications.

We claim:
 1. An improved spatial light modulator of the type which includes a generally planar light-reflective pixel which is deflectable out of a first, normal position into a plurality of second positions, light incident on the pixel being selectively modulated by being reflected from the pixel in selected directions depending on the position of the pixel, the position of the pixel being dependent on a selected characteristic of an electrical signal provided to an underlying address circuit, the deflection of the pixel storing potential mechanical energy in a pixel-supporting facility, which stored energy tends to return the pixel to the first position, wherein the improvement comprises:the pixel-supporting facility includinga first torsion member connected between the pixel and a first stationary post and defining a torsion axis, deflection of the pixel effecting its rotation about the torsional axis of the first member; and a second flexure member connected between the pixel and a second stationary post, deflection of the pixel effecting flexure of the second member.
 2. An improved modulator as set forth in claim 1, wherein:the points of connection of the members to the pixel are separated about the periphery of the pixel.
 3. An improved modulator as in claim 2, wherein:the pixel has a generally orthogonal profile, the first member is connected to the pixel at a mid-point of a first side thereof, and the second member is connected to the first side of the pixel.
 4. An improved modulator as in claim 3, wherein:the second member is connected to the pixel proximate the juncture of the first side of the pixel and a second side of the pixel.
 5. An improved modulator as in claim 2, wherein:the torsional axis of the first member is generally co-planar with the pixel, and the second member in its unflexed state is oriented so as to be generally co-planar with the pixel and perpendicular to the torsional axis of the first member.
 6. An improved modulator as in claim 5, wherein:the second member in its flexed state defines a curved surface, the surface of which is generally perpendicular to the torsional axis of the first member.
 7. An improved modulator as in claim 1, wherein:the characteristics of the first member are such that the deflection of the pixel out of the first position is predominantly rotational about the torsion axis, and the characteristics of the second member are such that the first position of the pixel is predominantly determined by the second member.
 8. An improved modulator as in claim 7, wherein:the characteristics of the members are such that the pixel is selectively deflectable out of the first position into a plurality of the second positions determined by the selected characteristic of the electrical signal.
 9. An improved modulator as in claim 1, wherein:the first and second members are respectively connected to separated said first and second posts.
 10. An improved modulator as in claim 1, wherein:the first and second members are connected to the same said posts.
 11. An improved modulator as in claim 1, wherein:the pixel is reinforced to maintain flatness.
 12. The improved modulator as in claim 11, wherein:the pixel is reinforced about a perimeter thereof.
 13. The improved modulator as in claim 12, wherein:the pixel is corrugated about the perimeter.
 14. The improved modulator as in claim 11, wherein the pixel is radially reinforced from proximate a midsection thereof. 